Hi dirtylarry,
See here:
https://www.hifiberry.com/build/documentation/dac-pro-external-power-supply/ and here:
https://www.hifiberry.com/build/documentation/dac-pro-connect-external-i2s-dacs/ These documents are really difficult to find, the search engines seem to miss them. There are issues with an UART conflict on the Pi 3B, you need to disable the on-board Bluetooth, see here:
#47313
janui